A conventional a multi-phase rotary machine control apparatus controls a multi-phase rotary machine by switching on and off a plurality of switching elements. It is proposed by the following patent documents to drive a rotary machine to maintain its rotation even if a part of a plurality of switching elements fails.    Patent document 1: JP 2005-304119A    Patent document 2: JP 6-98596A    Patent document 3: JP 2009-6963A (US 2010/0017063 A1)
According to patent document 1, a plurality of electric power supply sources such as inverters is provided. If any one of the power supply sources fails to operate normally, the power supply from such a failing power supply source to a group of a plurality of coils corresponding to the failing power supply source is stopped. The power supply sources, which are other than the failing power supply source and operating normally, are controlled to supply electric power to such a group of coils corresponding to the failing power supply source. However, the amount of electric power supplied to the rotary machine will be reduced by an amount, which the failing power supply source has supplied normally. If the electric power of the other normal power supply sources is increased to compensate the electric power for the reduction, the switching elements of the normal power supply sources generate more heat. Thus, the period, in which the normal power supply sources are allowed to output increased power for compensation, is limited to be short.
According to patent document 2, a target value of an armature current of each phase of an electric motor is calculated as a function of an inverse-proportion value and a direct proportion value. The inverse proportion value is inversely proportional to a sum of squares of instant values of induced voltages of all phases. The direct proportion value is directly proportional to an instant value of an induced voltage of each phase. The armature current of each phase is controlled in accordance with the calculated target value. In a single-phase inverter system, a current is supplied to each coil independently from other coils. A phase current command value is therefore readily calculated in inverse proportion to a sum of squares of induced voltages. In a three-phase inverter system for a three-phase motor for example, currents Iu, Iv and Iw supplied to phases U, V and W, respectively, are restricted to satisfy Iu+Iv+Iw=0. Under this restriction, the phase current command values need be calculated in inverse proportion to a sum of squares of induced voltages. If an off-failure arises in one of phases, for example U-phase, under the restriction and hence no current Iu is supplied, other phase currents Iv and Iw are restricted to satisfy Iv=−Iw. As a result, no torque is generated in some periods.
According to patent document 3, if an off-failure arises in one phase of a three-phase inverter system, phase current command values for the other two phases are calculated to satisfy Iu+Iv+Iw=0. The phase current command values are calculated by referring to a data table, which stores predetermined data as a function of an angular position of a rotor. If the off-failure arises, the same problem arises as in the patent document 2. Further, a memory of a large capacity is required to store the phase current command values as a data table to be referred to in case one of the three phases of the three-phase inverter system fails.